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The
headwaters of Goose Creek, a tributary of Chester Creek, run through a
narrowly wooded portion of the West Goshen Shopping Center property. Goose Creek has been visibly impacted (serious bank
erosion and water quality problems) by the shopping center and major
highway construction, which have significantly altered the stream's natural hydrology. Existing stormwater management on site is minimal and
includes surface drainage inlets and a sub-surface storm system designed
to convey runoff as quickly as possible to the wooded area and Goose
Creek stream. The site of Goshen Shopping Center also suffers
from flooding problems, which can interfere with shopping center
activity and result in costly interior store damages.
To remedy the current
conditions, the conceptual retrofit outlines techniques to manage on-site
and off-site stormwater volumes:
1) Subsurface infiltration bed/ Linear trenches under parking;
2) Bioretention basins/ Infiltration trenches; 3) Site
naturalization.
1)
Subsurface Infiltration Bed/ Linear Trenches Under Parking
It is recommended that a
large sub-surface infiltration bed or series of linear trenches be
installed under the existing parking areas in front of the retail spaces
in order to manage on-site and off-site stormwater runoff. The main
parking lot is approximately 5 acres in size, providing ample area to
reduce stormwater runoff volumes. It is recommended that this system be sized to
manage runoff from the parking area as well as runoff being conveyed to
the site from the inlets located on Paoli Pike.
This type of infiltration
system consists of a uniformly graded aggregate storage bed or trench
wrapped with geotextile fabric and finished with a porous or standard
asphalt surface. The system should be designed with a positive overflow
that allows it to fill up before overflowing to ensure maximum storage
capacity within the bed. Large infiltration systems provide an excellent
opportunity to manage the runoff (volume and peak rate) and to provide
for water quality control of the most pollutant-laden “first flush”
stormwater runoff discharges. The type of pavement used
will affect the design but the system is not dependent on the use of
porous asphalt. Porous pavement allows stormwater to drain directly
through the pavement; however, if standard impervious asphalt is used,
inlets can convey runoff into the sub-surface bed.
2) Bioretention Basins/ Infiltration Trenches
There are several
stormwater discharge points located on the north side of the property
(behind the stores), which convey runoff directly to the stream. It is
recommended that these outlet areas be retrofitted with bioretention
areas, where feasible, to slow and filter runoff before discharge to
Goose Creek. It is also recommended that roof leaders located in
the rear of the buildings be disconnected and diverted to subsurface
infiltration trenches/beds or to bioretention areas. A bioretention area should be designed as a shallow
depressed landscaped area with amended soils, and should be allowed to fill up to a
maximum depth of 6 to 10 inches before overflowing. The area should be
planted with native species tolerant of both wet and dry conditions.
This system would reduce runoff volume through infiltration and evapotranspiration. For a bioretention diagram by Cahill Associates,
Inc. click
here.
Installing infiltration trenches is another option.
An infiltration trench is a linear element consisting of a
stone-filled trench wrapped in geotextile. The surface of the trench can
be vegetated. Infiltration trenches effectively reduce runoff volume and
rate by providing stormwater storage and allowing for the infiltration
of runoff. For an infiltration trench diagram by Cahill
Associates, Inc. click
here.
3) Site Naturalization
The West Goshen Shopping
Center property is virtually 100 percent impervious. Much of the site’s
parking is under-utilized, especially areas located behind the retail
stores. Some of this impervious area is needed for delivery trucks and
fire access but the majority of the parking spaces are unnecessary and
are not used by consumers or employees. Therefore, it is recommended
that the site owner work with the Township to revise and reduce the
parking requirements, subsequently removing unused pavement and
restoring the land to native woodland areas. This will reduce runoff and
nonpoint source pollution, increase groundwater recharge and
evapotranspiration, and extend the existing riparian corridor located on
the north side of the shopping center.
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County:
Chester
Watershed: Chester Creek
The West Goshen Shopping
Center is located on Paoli Pike between the Route 202 Bypass and Turner
Lane in West Goshen Township, Chester County. The site encompasses
approximately 32 acres and is surrounded by commercial areas, mixed
density residential housing, and major highways and ramps. The site is
virtually 100 percent impervious, occupied primarily by a strip mall
with parking in both the front and rear of the retail spaces.
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